The scientists of the time were certainly fantastic engineers, making huge strides in the advancement of all technologies, but this thing was really a beast, despite the simplicity of the concept, which is more or less this, on a reduced scale:

You have a gun. You pull the trigger, the firing pin ignites the powder in the shell, bullet shoots out. Fine.

Now, add additional charges as the bullet is traveling through the barrel, timed to go off just after it passes, thus increasing its velocity.

The theory behind this monstrosity is that your initial blast can only be so big; enormous shells are impractical, and might blow the weapon apart anyway. So--do it in stages. Simple.

Of course, if you want to get your bullet into the stratosphere (which they did, knowing that the thinner air would further increase the range), and if you want to make more than a pinprick in the face of London, everything will have to be dramatically increased in size.

The project didn't really get underway until 1943, when a 20mm prototype was successfully demonstrated at Miedzyzdroje, in Poland. Going against the advice of his military commanders, Hitler himself ordered the construction of no less than fifty of the guns, to be placed in strategically located underground bunkers along France's coast.

They never got a decent shot off. The French Resistance leaked word to the Allies almost as soon as construction began; it's not easy to conceal such a massive undertaking, especially when the main compononent is 150 meters long.

The bombs rode all the way down, 30 meters, to the main floor of the complex. Anyone working at the time was killed. Hitler, rushing whatever he had left into service, managed to get two smaller versions operational to no avail. No results were recorded.

The project to bomb London using the V-3 was thus scrapped, but tests with shorter-barreled versions were continued under SS-Gruppenführer Hans Kammler. He ordered the project to be intensified to be ready for action in late fall 1944.

Two 150 mm caliber versions with 12 pairs of side chambers at 90° and a barrel length of some 50 meters were constructed some 43 km east of Luxembourg (the city). These were subordinated to the 550 man strong Heeres Artillerie Abteilung (army artillery unit) 705 under Hauptmann Patzig.

The shorter V-3's were almost complete by the time the Germans started their offensive in the Ardennes, but procuring proper ammunition for the guns was difficult due to the disrupted railway lines. It was decided to use high explosive charges from various available ammunition, amounting to 7-9 kg of payload for each 95-kg shell. The initial propellant charge was 5 kg, and the secondary charges totalled 68 kg.

Luxembourg was their designated target. The shelling started on December 30th, 1944. Between this date and February 22nd, 1945, when the advancing Allied forces made further operations impossible, the two V-3's fired a total of 183 projectiles at Luxembourg, with 143 hitting the city.

The V-3 shelling of Luxembourg claimed a total of 10 (civilian) lives and wounded 35.